Centrifugal fan

ABSTRACT

A centrifugal fan includes a motor including a rotor, an impeller fixed to the rotor to rotate together with the rotor, a circuit board electrically connected to the motor, and a casing to house the motor, the impeller, and the circuit board. The impeller includes blade portions spaced apart from one another in a circumferential direction and extending radially outward, an annular upper shroud to join axially upper portions of the blade portions, and an annular lower shroud to join axially lower portions of the blade portions. The casing includes an upper casing to cover an axially upper side of the impeller. The upper casing includes a tubular portion radially opposite to the upper shroud with a first gap therebetween, and an annular disk-shaped portion extending radially inward from the tubular portion and axially opposite to an axially upper end portion of the upper shroud with a second gap therebetween. The disk-shaped portion, including an air inlet defined radially inside thereof, includes an axially recessed portion recessed axially upward in a lower surface thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to Japanese Patent Application No. 2018-041586 filed on Mar. 8, 2018. The entire contents of this application are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present disclosure relates to a centrifugal fan.

2. Description of the Related Art

In a known centrifugal fan, an impeller including a plurality of blades arranged in a circumferential direction is housed between an upper plate and a lower plate of a casing. In the centrifugal fan, rotation of the impeller causes air to be sucked in through an air inlet defined in the upper plate of the casing, and to be blown out from an outer circumference of the impeller. An upper end of a cylindrical portion of the impeller is arranged to project from the casing, and is arranged to define a curved portion in the shape of a bell mouth which curves outwardly. Near the outer circumference of the impeller, an air blown out from the impeller flows into a gap between the impeller and the upper plate of the casing. This air flows backward toward the air inlet, and meets an air flow sucked in through the air inlet.

However, the meeting of the air flows described above may cause noise to be generated at the air inlet due to an occurrence of turbulence. Further, a reduction in air-blowing performance of the centrifugal fan may occur due to the intake of air through the air inlet being reduced by an occurrence of turbulence.

SUMMARY OF THE INVENTION

A centrifugal fan according to an example embodiment of the present disclosure includes a motor including a rotor to rotate about a central axis extending in a vertical direction, an impeller fixed to the rotor to rotate together with the rotor, a circuit board electrically connected to the motor, and a casing to house the motor, the impeller, and the circuit board. The impeller includes a plurality of blade portions spaced apart from one another in a circumferential direction and extending radially outward, an annular upper shroud to join at least portions of axially upper portions of the blade portions, and an annular lower shroud to join at least portions of axially lower portions of the blade portions. The casing includes an upper casing to cover an axially upper side of the impeller. The upper casing includes a tubular portion radially opposite to the upper shroud with a first gap therebetween, and an annular disk-shaped portion extending radially inward from the tubular portion and axially opposite to an axially upper end portion of the upper shroud with a second gap therebetween. The disk-shaped portion includes an air inlet defined radially inside thereof. The disk-shaped portion includes at least one axially recessed portion recessed axially upward in a lower surface of the disk-shaped portion.

The above and other elements, features, steps, characteristics and advantages of the present disclosure will become more apparent from the following detailed description of the example embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external perspective view illustrating the overall structure of a centrifugal fan according to an example embodiment of the present disclosure.

FIG. 2 is a vertical sectional view of a centrifugal fan according to an example embodiment of the present disclosure.

FIG. 3 is a perspective view of an impeller according to an example embodiment of the present disclosure as viewed from above in an axial direction.

FIG. 4 is a partial sectional view of a centrifugal fan according to an example embodiment of the present disclosure.

FIG. 5 is a perspective view illustrating an exemplary structure of an upper casing according to a preferred embodiment of the present disclosure.

FIG. 6A is a bottom view illustrating an example of at least one axially recessed portion according to an example embodiment of the present disclosure.

FIG. 6B is a bottom view illustrating another example of the at least one axially recessed portion according to a preferred embodiment of the present disclosure.

FIG. 7 is a perspective view illustrating another example structure of the upper casing according to an example embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, example embodiments of the present disclosure will be described with reference to the accompanying drawings. It is assumed herein that a rotation axis of a motor 30 of a centrifugal fan 100 is referred to as a central axis P, and that a direction parallel to the central axis P is referred to by the term “axial direction”, “axial”, or “axially”. It is also assumed herein that a direction leading from a circuit board 40 to an impeller 10 is referred to as an axially upward direction, while a direction leading from the impeller 10 to the circuit board 40 is referred to as an axially downward direction. It is also assumed herein that a surface of any structural element which faces upward in an axial direction is referred to as an upper surface thereof, while a surface of any structural element which faces downward in the axial direction is referred to as a lower surface thereof. It is also assumed herein that an end portion of any structural element on an axially upper side is referred to as an “axially upper end portion”, and that the position of the end portion thereof on the axially upper side is referred to as an “axially upper end”. Further, it is assumed herein that an end portion of any structural element on an axially lower side is referred to as an “axially lower end portion”, and that the position of the end portion thereof on the axially lower side is referred to as an “axially lower end”.

It is also assumed herein that a direction in which a straight line perpendicular to the central axis P extends is referred to by the term “radial direction”, “radial”, or “radially”. It is also assumed herein that a direction of the central axis P in a radial direction is referred to as a radially inward direction, while a direction away from the central axis P in the radial direction is referred to as a radially outward direction. It is assumed herein that a side surface of any structural element which faces in a radial direction is referred to as a “radial side surface”. In particular, a side surface of any structural element which faces radially inward is referred to as a “radially inner surface”, while a side surface of any structural element which faces radially outward is referred to as a “radially outer surface”. It is also assumed herein that an end portion of any structural element in a radial direction is referred to as a “radial end portion”, and that the position of the end portion thereof in the radial direction is referred to as a “radial end”. In particular, an end portion of any structural element on a radially inner side is referred to as a “radially inner end portion”, and the position of the end portion thereof on the radially inner side is referred to as a “radially inner end”. Further, an end portion of any structural element on a radially outer side is referred to as a “radially outer end portion”, and the position of the end portion thereof on the radially outer side is referred to as a “radially outer end”.

A direction along a rotation direction about the central axis P is sometimes referred to by the term “circumferential direction”, “circumferential”, or “circumferentially”. In addition, when viewed from below in the axial direction, a clockwise circumferential direction is referred to as a “forward rotation direction Rdf”, and a counterclockwise circumferential direction is referred to as a “backward rotation direction Rdb”. A side surface of any structural element which faces in the rotation direction is referred to as a “rotation-direction side surface”. In particular, a side surface of any structural element which faces forward in the rotation direction, i.e., in the forward rotation direction Rdf, is referred to as a “forward rotation-direction side surface”, while a side surface of any structural element which faces backward in the rotation direction, i.e., in the backward rotation direction Rdb, is referred to as a “backward rotation-direction side surface”.

It should be noted that the above definitions of the directions, the surfaces, the end portions, the positions thereof, etc., are not meant to restrict in any way relative positions, directions, etc., of different members or portions of a centrifugal fan according to any preferred embodiment of the present disclosure when actually installed in a device.

Also note that the term “parallel” as used herein includes both “parallel” and “substantially parallel”. Also note that the term “perpendicular” as used herein includes both “perpendicular” and “substantially perpendicular”.

FIG. 1 is an external perspective view illustrating the overall structure of a centrifugal fan 100 according to a preferred embodiment of the present disclosure. FIG. 2 is a vertical sectional view of the centrifugal fan 100 according to a preferred embodiment of the present disclosure. In FIG. 1, the centrifugal fan 100 as viewed from above in the axial direction is shown.

Referring to FIGS. 1 and 2, the centrifugal fan 100 includes a casing 1, an impeller 10, a motor 30, and a circuit board 40.

The casing 1 is arranged to house the impeller 10, the motor 30, and the circuit board 40. The casing 1 includes an upper casing 2 and a lower casing 3. Each of the upper casing 2 and the lower casing 3 is made of, for example, a resin. Note, however, that each of the upper casing 2 and the lower casing 3 may alternatively be made of a material other than the resin, such as, for example, a metal. The upper casing 2 and the lower casing 3 may be made of either the same material or different materials.

The upper casing 2 is arranged to cover an axially upper side of the impeller 10. Provision of the upper casing 2 reduces the likelihood of an occurrence of turbulence near an upper shroud 15, which will be described below, and enables the centrifugal fan 100 to efficiently send air in a centrifugal direction. The upper casing 2 includes an air inlet 2 a opposite to a radially central portion of the impeller 10. The air inlet 2 a is circular. Note, however, that the shape of the air inlet 2 a may not necessarily be circular. The structure of the upper casing 2 will be described below.

In the present preferred embodiment, an air outlet 2 b is defined between the upper casing 2 and the lower casing 3, and the air outlet 2 b is arranged to extend over the entire circumferential extent of the casing 1. Note, however, that the air outlet 2 b is not limited to this example, and that the air outlet 2 b may alternatively be arranged to extend over only a part of the entire circumferential extent of the casing 1.

The lower casing 3 is arranged on the axially lower side of the impeller 10. The lower casing 3 is rectangular in a plan view as viewed in the axial direction, and is arranged to have substantially the same size as the upper casing 2. The lower casing 3 includes a board housing portion 4 and a flange portion 5.

The board housing portion 4 is recessed axially downward, and is arranged to house the circuit board 40. In the present preferred embodiment, the board housing portion 4 is arranged to have the same shape as the circuit board 40, and be slightly greater in radial dimension than the circuit board 40. Note, however, that the shape of the board housing portion 4 may not necessarily be the same as the shape of the circuit board 40. The motor 30 is arranged at a radially central portion of the board housing portion 4, and a portion of the circuit board 40 is arranged to project radially outward relative to the motor 30.

The flange portion 5 is arranged to extend radially outward from a radially outer end portion of the board housing portion 4. In the present preferred embodiment, the upper casing 2 is, for example, screwed to the flange portion 5, so that the upper casing 2 is fixed to the lower casing 3. Note that the upper casing 2 may alternatively be fixed to the lower casing 3 by another fixing method, such as, for example, use of an adhesive.

The impeller 10 is fixed to a rotor 32, which will be described below, of the motor 30, and is arranged to rotate together with the rotor 32. The structure of the impeller 10 will be described below.

The motor 30 includes the rotor 32. The rotor 32 is arranged to rotate about the central axis P, which extends in a vertical direction. More specifically, the rotor 32 is arranged to rotate counterclockwise about the central axis P when viewed from above in the axial direction. The rotor 32 is arranged axially above and radially outside of a stator 31. The rotor 32 is in the shape of a cup opening downward in the axial direction. The impeller 10 is arranged radially outside of the rotor 32, and the impeller 10 is fixed to the rotor 32. A shaft 33 is arranged radially inside of the rotor 32, and the shaft 33 is fixed to the rotor 32. A rotor magnet 36 is fixed to a radially inner surface of the rotor 32. In the present preferred embodiment, the rotor magnet 36 is a single annular magnet. A radially inner surface of the rotor magnet 36 includes north and south poles arranged to alternate with each other in a circumferential direction. Note that, instead of the single annular magnet, a plurality of magnets may alternatively be arranged on the radially inner surface of the rotor 32.

The motor 30 further includes the stator 31, the shaft 33, bearing portions 34, and a bearing holding portion 35.

The shaft 33 is a columnar member arranged to extend along the central axis P. A metal, such as stainless steel, for example, is used as a material of the shaft 33. An axially upper end portion of the shaft 33 is arranged axially above an upper one of the bearing portions 34. The axially upper end portion of the shaft 33 is arranged to pass through a rotor hole passing through the rotor 32 in the axial direction along the central axis P, and is fixed to the rotor 32.

The bearing portions 34 are arranged to support the shaft 33 such that the shaft 33 is capable of rotating about the central axis P. In the present preferred embodiment, the number of bearing portions 34 is two, and the two bearing portions 34 are arranged one above the other in the vertical direction. Each of the two bearing portions 34 is defined by a ball bearing. Note that the number of bearing portions 34 and the types of the bearing portions 34 may be changed appropriately.

The bearing holding portion 35 is arranged to support the stator 31 on the radially outer side, and support the bearing portions 34 on the radially inner side. A metal, such as stainless steel or brass, for example, is used as a material of the bearing holding portion 35. Note, however, that the bearing holding portion 35 may not necessarily be made of a metal, but may alternatively be made of a resin. The bearing holding portion 35 is arranged to extend in the axial direction around the central axis P to assume a cylindrical shape. An axially lower end portion of the bearing holding portion 35 is inserted into a circular hole centered on the central axis P and defined in the lower casing 3, and is fixed to the lower casing 3.

The stator 31 is an armature arranged to generate magnetic flux in accordance with electric drive currents. The stator 31 includes a stator core, an insulator, and coils.

The stator core is a magnetic body. The stator core is defined by, for example, laminated electromagnetic steel sheets. The stator core includes a core back in the shape of a circular ring, and a plurality of teeth. The core back is fixed to a radially outer surface of the bearing holding portion 35. The teeth are arranged to project radially outward from the core back. The insulator is an insulating body. A resin, for example, is used as a material of the insulator. The insulator is arranged to cover at least a portion of the stator core, in particular, the teeth. Each coil includes a conducting wire wound around a corresponding one of the teeth with the insulator therebetween.

As a result of supply of the electric drive currents to the stator 31, a rotary torque is generated between the rotor magnet 36 and the stator 31. As a result, the rotor 32 is caused to rotate with respect to the stator 31. Accordingly, the impeller 10, which is fixed to the rotor 32, is caused to rotate about the central axis P. The motor 30 illustrated in FIG. 2 is an outer-rotor motor in which the rotor 32 is arranged radially outside of the stator 31. Note, however, that the motor 30 may alternatively be an inner-rotor motor in which the rotor 32 is arranged radially inside of the stator 31.

The circuit board 40 is electrically connected to the motor 30. The circuit board 40 is supported on an axially lower portion of the motor 30. The circuit board 40 is arranged in the board housing portion 4 of the lower casing 3. The circuit board 40 is arranged substantially perpendicularly to the central axis P between the lower casing 3 and the stator 31. The circuit board 40 is fixed to, for example, the insulator. An electrical circuit to supply the electric drive currents to the coils is mounted on the circuit board 40. End portions of the conducting wires included in the coils are electrically connected to terminals arranged on the circuit board 40.

Next, the structure of the impeller 10 will now be described below. FIG. 3 is a perspective view of the impeller 10 as viewed from above in the axial direction. FIG. 4 is a partial sectional view of the centrifugal fan 100 according to a preferred embodiment of the present disclosure.

Referring to FIGS. 3 and 4, the impeller 10 includes a plurality of blade portions 13, the upper shroud 15, and a lower shroud 17. In addition, the impeller 10 further includes a boss portion 11 and a wall portion 19. In the present preferred embodiment, the boss portion 11, the blade portions 13, the upper shroud 15, the lower shroud 17, and the wall portion 19 are made of the same resin material.

The boss portion 11 is tubular, and is fixed to the rotor 32. In the present preferred embodiment, the boss portion 11 is cylindrical, and is fixed to a radially outer surface of the rotor 32 on the axially upper side of the motor 30. The boss portion 11 is fixed to the rotor 32 through, for example, press fitting or adhesion. In more detail, the boss portion 11 includes, at an axially upper end thereof, a projecting portion in the shape of a circular ring and arranged to project radially inward. The projecting portion is arranged on the axially upper side of the rotor 32. The projecting portion is arranged so that a weight member for balance adjustment can be arranged thereon, for example. Note, however, that this example is not essential to the present invention, and that the projecting portion may not be provided. In the case where the projecting portion is not provided, the impeller 10 may be fixed to only the radially outer surface of the rotor 32.

The boss portion 11 is arranged to radially overlap with the blade portions 13 and the rotor 32. Each of an axially lower end of the rotor 32 and an axially lower end of the boss portion 11 is arranged at a level higher than that of an axially lower end of each blade portion 13. Further, the boss portion 11 is arranged to have an axial dimension smaller than that of the impeller 10 as a whole. Thus, a reduced thickness of the centrifugal fan 100 is achieved.

The blade portions 13 are arranged apart from one another in the circumferential direction on the radially outer side of the boss portion 11, and are arranged to extend radially outward. In the present preferred embodiment, each blade portion 13 is arranged radially opposite to the boss portion 11 with a gap therebetween. Note, however, that each blade portion 13 may alternatively be arranged to be in contact with the boss portion 11. The blade portions 13 are arranged to extend radially outward in a radial manner while slanting in a direction opposite to the rotation direction of the centrifugal fan 100 in a plan view. Note that the blade portions 13 may be arranged to extend radially outward in any desired direction, and that a portion of any blade portion 13 may be arranged to extend in the same direction as the rotation direction or extend perpendicularly to the rotation direction. Also note that, although the blade portions 13 are arranged at regular intervals in the circumferential direction in the present preferred embodiment, the blade portions 13 may not necessarily be arranged at regular intervals.

The upper shroud 15 is annular, and is arranged to join at least portions of axially upper portions of the blade portions 13. In the present preferred embodiment, the upper shroud 15 is substantially in the shape of a truncated cone whose inside diameter decreases in the axially upward direction, and is arranged to join axially upper end portions of the blade portions 13.

The lower shroud 17 is annular, and is arranged to join at least portions of axially lower portions of the blade portions 13. The lower shroud 17 includes a slanting portion 171 and a flat portion 172. The slanting portion 171 is substantially in the shape of a truncated cone whose inside diameter decreases in the axially upward direction, and is arranged to slant axially downward while extending radially outward. In more detail, a radially inner end of the slanting portion 171 is connected to a radially outer surface of the boss portion 11. The slanting portion 171 is arranged to obliquely extend axially downward from the junction with the boss portion 11. An air flow coming in through the air inlet 2 a to travel in the axial direction is efficiently sent toward the air outlet 2 b through the slanting portion 171. The flat portion 172 is in the shape of a circular ring, and is arranged to extend radially outward continuously from a radially outer end portion of the slanting portion 171. The flat portion 172 is arranged to join axially lower end portions of the blade portions 13, and is capable of guiding an air flow which had traveled in the axial direction but is now traveling radially due to the slanting portion 171 toward the air outlet 2 b.

In the present preferred embodiment, the upper shroud 15 and the lower shroud 17 are joined to each other in the vertical direction by the blade portions 13.

The wall portion 19 is annular, and is arranged to extend from the upper shroud 15 toward the upper casing 2. In more detail, the wall portion 19 is in the shape of a cylinder extending axially upward in the present preferred embodiment. An axially upper portion of the wall portion 19 is housed in a wall housing portion 2 c, which will be described below, of the upper casing 2. Further, the wall portion 19 is arranged radially outward of a disk-shaped portion 22, which will be described below, of the upper casing 2, and radially inward of a radially outer end of the upper shroud 15.

Next, the structure of the upper casing 2 will now be described below with reference to FIGS. 4, 5, 6A, 6B, and 7. FIG. 5 is a perspective view illustrating an exemplary structure of the upper casing 2. FIG. 6A is a bottom view of the upper casing 2, illustrating an example of at least one axially recessed portion 22 a, which will be described below. FIG. 6B is a bottom view of the upper casing 2, illustrating another example of the at least one axially recessed portion 22 a. FIG. 7 is a perspective view illustrating another exemplary structure of the upper casing 2. In FIG. 5, a section in the lower diagram represents a section taken along line B-B in the upper diagram. In FIG. 7, a section in the lower diagram represents a section taken along line C-C in the upper diagram. In each of FIGS. 5 to 7, the upper casing 2 as viewed from below in the axial direction is shown. When viewed from below in the axial direction, the rotation direction of each of the impeller 10 and the rotor 32 is a clockwise direction about the central axis P.

The upper casing 2 includes a tubular portion 21 and the disk-shaped portion 22. In addition, the upper casing 2 further includes four corner portions 23 and the wall housing portion 2 c.

The tubular portion 21 is arranged radially opposite to the upper shroud 15 of the impeller 10 with a first gap therebetween. In more detail, the tubular portion 21 is arranged radially opposite to an axially upper portion of the upper shroud 15 with the first gap, having a radial width dr, therebetween.

The disk-shaped portion 22 is arranged to extend radially inward from a radially inner end portion of the tubular portion 21. The disk-shaped portion 22 is annular, and is arranged axially opposite to an axially upper end portion of the upper shroud 15 with a second gap therebetween. In more detail, a lower surface of the disk-shaped portion 22 is arranged axially opposite to the axially upper end portion of the upper shroud 15 with the second gap, having an axial width da, therebetween.

The disk-shaped portion 22 has the air inlet 2 a and the axially recessed portion(s) 22 a. The air inlet 2 a is defined radially inside of the disk-shaped portion 22. A radially inner surface of the disk-shaped portion 22 is a curved surface being convex axially upward and radially inward, and is arranged to extend radially outward while extending axially upward. The radially inner surface of the disk-shaped portion 22 defines a bell mouth of the air inlet 2 a. The axially recessed portion(s) 22 a is recessed axially upward in the lower surface of the disk-shaped portion 22.

Air sucked in through the air inlet 2 a of the upper casing 2 whirls in the circumferential direction within the casing 1 due to the rotation of the impeller 10, and is then discharged through the air outlet 2 b defined between the upper casing 2 and the lower casing 3. The upper shroud 15 and the lower shroud 17 improve fan efficiency of the centrifugal fan 100 by efficiently guiding the air sucked into the casing 1 through the air inlet 2 a to the air outlet 2 b.

A portion of an air flow which is sent radially outward from the impeller 10 flows into a gap between the upper casing 2 and the upper shroud 15. In more detail, this portion of the air flow flows into the first gap between the upper shroud 15 and the tubular portion 21 of the upper casing 2, and flows toward the second gap between the axially upper end portion of the upper shroud 15 and the disk-shaped portion 22. Here, the axial width da of the second gap is increased by the axially recessed portion(s) 22 a. Accordingly, the flow speed of the air flow flowing into the second gap is reduced on the axially lower side of the axially recessed portion(s) 22 a. That is, the flow speed of an air flow flowing into the air inlet 2 a out of the second gap is reduced. This reduces the likelihood that turbulence will occur when this air flow meets an air flow sucked in through the air inlet 2 a. Accordingly, a reduction in flow rate of air sucked in through the air inlet 2 a and generation of noise due to an occurrence of turbulence can be reduced. Thus, the centrifugal fan 100 is excellent in air-blowing performance, and produces relatively little noise.

Here, as illustrated in FIG. 6A, the axially recessed portion(s) 22 a may include a plurality of axially recessed portions 22 a arranged in the circumferential direction. In this case, the above-described effect of the reduced likelihood of an occurrence of turbulence at the air inlet 2 a can be obtained evenly in the circumferential direction. In addition, in an interior of each axially recessed portion 22 a, a circumferential flow of air is interrupted by a circumferential inside surface of the axially recessed portion 22 a. This reduces the flow speed of the air flow in the circumferential direction. This contributes to further reducing the flow speed of the air flow flowing into the air inlet 2 a out of the second gap.

Further, in the case where the plurality of axially recessed portions 22 a are provided, the axially recessed portions 22 a are preferably arranged at regular intervals in the circumferential direction. The above-described effect can be obtained evenly in the circumferential direction when the plurality of axially recessed portions 22 a are equally spaced from one another. Note, however, that this example is not essential to the present invention, and that the axially recessed portions 22 a may alternatively be arranged at irregular intervals in the circumferential direction, instead of being equally spaced from one another.

Alternatively, as illustrated in FIG. 6B, the axially recessed portion(s) 22 a may be a single axially recessed portion 22 a being annular when viewed in the axial direction. In this case, the above-described effect of the reduced likelihood of an occurrence of turbulence at the air inlet 2 a can be obtained evenly in the circumferential direction. This contributes to further reducing the flow speed of the air flow flowing into the air inlet 2 a out of the second gap.

In addition, the radial width dr of the first gap is preferably arranged to be smaller than the axial width da of the second gap. More specifically, the first gap between the tubular portion 21 and the upper shroud 15 is preferably arranged to be narrower than the second gap between the disk-shaped portion 22 and the axially upper end portion of the upper shroud 15. Tolerances of the axial dimensions of members that are arranged in the axial direction, such as the lower casing 3, the impeller 10, and the upper casing 2, for example, are larger than tolerances of the radial dimensions of members that are arranged in the radial direction. According to the above arrangement, a reduction in the flow speed of the air flow flowing into the air inlet 2 a out of the second gap can be achieved by the axial width da of the second gap being greater than the radial width dr of the first gap. In addition, the narrowing of the first gap makes it possible to secure the radial width dr of the first gap without excessively reducing the diameter of the air inlet 2 a.

Next, the tubular portion 21 includes a plurality of radially protruding portions 211. Each radially protruding portion 211 is arranged to project radially inward and extend in the axial direction at a radially inner surface of the tubular portion 21.

The radially protruding portions 211 are preferably arranged at regular intervals in the circumferential direction. The above-described effect of further reducing the reduction in the flow rate and the generation of noise can be obtained evenly in the circumferential direction when the radially protruding portions 211 are equally spaced from one another. Note, however, that this example is not essential to the present invention, and that the radially protruding portions 211 may alternatively be arranged at irregular intervals in the circumferential direction, instead of being equally spaced from one another. When the radially protruding portions 211 are arranged at irregular intervals in the circumferential direction, acoustic frequencies generated by interference between the upper shroud 15 and the radially protruding portions 211 during the rotation of the impeller 10 are distributed, resulting in reduced generation of noise.

An axially upper end portion of each radially protruding portion 211 is preferably connected to the lower surface of the disk-shaped portion 22. In this case, the second gap can be divided by the radially protruding portions 211 in the circumferential direction. Thus, a circumferential flow of air flowing in the second gap is restrained by each radially protruding portion 211. This contributes to further reducing the flow speed of the air flow flowing into the air inlet 2 a out of the second gap.

In addition, the axially recessed portion(s) 22 a is arranged to circumferentially overlap with the radially protruding portions 211. In this case, the effect of reducing the flow speed of an air flow on the axially lower side of the axially recessed portion(s) 22 a, and the effect of restraining the circumferential flow of air flowing in the second gap by each radially protruding portion 211, can be combined. This combination contributes to more effectively reducing the flow speed of the air flow flowing into the air inlet 2 a out of the second gap.

In addition, the axially upper end portion of each radially protruding portion 211 is preferably housed in the axially recessed portion(s) 22 a. More preferably, the axially upper end portion of each radially protruding portion 211 is connected to an inner bottom surface, which faces axially downward, of the axially recessed portion(s) 22 a. In this case, a circumferential flow of air in the axially recessed portion(s) 22 a can be restrained by each radially protruding portion 211. This contributes to more effectively reducing the flow speed of the air flow flowing into the air inlet 2 a out of the second gap. Further, when the axially upper end portion of each radially protruding portion 211 is connected to the inner bottom surface of the axially recessed portion(s) 22 a, the circumferential flow of air in the axially recessed portion(s) 22 a can be restrained by each radially protruding portion 211, and, in addition, an improvement in strength of each radially protruding portion 211 is achieved. Note, however, that the above examples are not essential to the present invention, and that the axially upper end portion of each radially protruding portion 211 may not be in contact with the inner bottom surface of the axially recessed portion(s) 22 a, or be housed in the axially recessed portion(s) 22 a.

As illustrated in FIG. 5, when viewed in the axial direction, a backward rotation-direction side surface of each radially protruding portion 211 is preferably an inclined surface extending in the forward rotation direction Rdf while extending radially inward, or a curved surface being convex radially inward and in the backward rotation direction Rdb. Here, the backward rotation-direction side surface, the forward rotation direction Rdf, and the backward rotation direction Rdb are defined on the basis of the rotation direction of the impeller 10.

According to a simulation result, when the backward rotation-direction side surface of each radially protruding portion 211 is the inclined surface or the curved surface as described above, noise generated will be smaller than in the case where the backward rotation-direction side surface of each radially protruding portion 211 is a flat surface parallel to a radial direction. When the above backward rotation-direction side surface is the inclined surface or the curved surface, the likelihood of a rapid flow change of an air flow near each radially protruding portion 211 will be lower than in the case where the backward rotation-direction side surface is a flat surface parallel to a radial direction. Accordingly, when the above backward rotation-direction side surface is the inclined surface or the curved surface as described above, the reduction in the flow rate of air sucked in through the air inlet 2 a and the generation of noise due to an occurrence of turbulence can be further reduced.

Note, however, that the backward rotation-direction side surface of each radially protruding portion 211 is not limited to the above example, but may alternatively be a flat surface parallel to a radial direction when viewed in the axial direction as illustrated in FIG. 7. Here, the backward rotation-direction side surface is defined on the basis of the rotation direction of the impeller 10.

The corner portions 23, which are arranged to extend radially outward, are arranged on a radially outer end portion of the tubular portion 21. In the present preferred embodiment, each corner portion 23 is fixed to the flange portion 5.

In addition, the wall housing portion 2 c, which is arranged to house at least a portion of the wall portion 19, is defined in a lower surface of the upper casing 2. The wall housing portion 2 c is annular, and is recessed axially upward. As illustrated in FIG. 4, the wall housing portion 2 c is arranged radially outward of the disk-shaped portion 22. In this case, because the wall housing portion 2 c and the wall portion 19 are arranged radially outward of the disk-shaped portion 22, an annular labyrinth structure defined by the wall portion 19 housed in the wall housing portion 2 c can be defined in a gap between the upper shroud 15 and the upper casing 2. This leads to a reduced flow rate of air flowing toward the first gap having the radial width dr.

In addition, as illustrated in FIG. 4, the wall housing portion 2 c is arranged radially inward of the radially outer end of the upper shroud 15. That is, the wall housing portion 2 c and the wall portion 19 are arranged radially inward of the radially outer end of the upper shroud 15. This makes it harder for an air flow to flow into the gap between the upper shroud 15 and the upper casing 2. Accordingly, a further reduction in the flow rate of air flowing out into the air inlet 2 a through the above gap is achieved.

The various technical features disclosed herein may be modified in a variety of manners within the scope of the technical ingenuity thereof. Note that any possible combination of the preferred embodiments and the modifications thereof described herein falls within the scope and spirit of the present disclosure.

Preferred embodiments of the present disclosure are applicable to, for example, centrifugal fans for use in air conditioners, range hood fans, duct fans, heat exchanger units, etc., and centrifugal fans used for paper suction in printers.

Features of the above-described preferred embodiments and the modifications thereof may be combined appropriately as long as no conflict arises.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims. 

What is claimed is:
 1. A centrifugal fan comprising: a motor including a rotor to rotate about a central axis extending in a vertical direction; an impeller fixed to the rotor to rotate together with the rotor; a circuit board electrically connected to the motor; and a casing to house the motor, the impeller, and the circuit board; wherein the impeller includes: a plurality of blade portions spaced apart from one another in a circumferential direction and extending radially outward; an annular upper shroud to join at least portions of axially upper portions of the blade portions; and an annular lower shroud to join at least portions of axially lower portions of the blade portions; the casing includes an upper casing to cover an axially upper side of the impeller; the upper casing includes: a tubular portion radially opposite to the upper shroud with a first gap therebetween; and an annular disk-shaped portion extending radially inward from the tubular portion and axially opposite to an axially upper end portion of the upper shroud with a second gap therebetween; the disk-shaped portion includes an air inlet defined radially inside thereof; and the disk-shaped portion includes at least one axially recessed portion recessed axially upward in a lower surface of the disk-shaped portion.
 2. The centrifugal fan according to claim 1, wherein the at least one axially recessed portion includes a plurality of axially recessed portions arranged in the circumferential direction.
 3. The centrifugal fan according to claim 1, wherein the at least one axially recessed portion is annular when viewed in an axial direction.
 4. The centrifugal fan according to claim 1, wherein the tubular portion includes a plurality of radially protruding portions each of which projects radially inward and extends in an axial direction at a radially inner surface of the tubular portion; and an axially upper end portion of each radially protruding portion is connected to the lower surface of the disk-shaped portion.
 5. The centrifugal fan according to claim 4, wherein, when viewed in the axial direction, a backward rotation-direction side surface of each radially protruding portion is an inclined surface extending in a forward rotation direction while extending radially inward, or a curved surface that is convex radially inward and in a backward rotation direction, the backward rotation-direction side surface, the forward rotation direction, and the backward rotation direction being defined on a basis of a rotation direction of the impeller.
 6. The centrifugal fan according to claim 4, wherein the radially protruding portions are arranged at regular intervals in the circumferential direction.
 7. The centrifugal fan according to claim 4, wherein the at least one axially recessed portion circumferentially overlaps with the radially protruding portions.
 8. The centrifugal fan according to claim 4, wherein the axially upper end portion of each radially protruding portion is housed in the at least one axially recessed portion.
 9. The centrifugal fan according to claim 1, wherein the first gap has a radial width smaller than an axial width of the second gap.
 10. The centrifugal fan according to claim 1, wherein the impeller further includes an annular wall portion extending from the upper shroud toward the upper casing; a lower surface of the upper casing includes an annular wall housing portion to house at least a portion of the wall portion, and recessed axially upward; and each of the wall portion and the wall housing portion is located radially outward of the disk-shaped portion.
 11. The centrifugal fan according to claim 10, wherein each of the wall portion and the wall housing portion is located radially inward of a radially outer end of the upper shroud. 